1. Field of the Invention
This invention relates to an improvement in the creep resistance of melt-fabricable fluoropolymers, and more particularly to an additive to the fluoropolymer that increases its creep resistance.
2. Description of Related Art
U.S. Pat. No. 4,163,742 (Mansure) discloses the blending of carbon fibers with melt-fabricable tetrafluoroethylene copolymer resin to improve resistance to creep. The copolymer resin has a melt viscosity of 104 to 107 poises at 380° C., which corresponds to a melt flow rate at 372° C. of about 0.1 to 50 g/10 min. and the fibers have an average length of at least 2 mm. The blending process involves the mixing together of the resin in particle form with the carbon fibers in a mixture of water and a water-miscible organic solvent, followed by filtering, drying to form a filter cake of the resin/fiber blend, and compression molding. Blend compositions containing up to 50 wt % carbon fiber are disclosed. U.S. Pat. No. 4,414,356 (Michel) discloses the practice of the same blending and compression molding processes, but requires the carbon fibers also having an average length of at least 2 mm to be free of surface treatment.
U.S. Pat. No. 4,422,992 (Michel) discloses that the Mansure process does not lend itself to acceptable continuous commercial production. The Michel process involves the co-feeding of (a) the melt-fabricable tetrafluoroethylene copolymer having the melt flow characteristics disclosed in Mansure and (b) continuous carbon fiber into a twin screw extruder, wherein the extruder chops the fiber into 1.5 to 3 mm lengths. Shorter fibers are disclosed to compromise the reinforcing characteristics of the fiber. The extruder melt blends the chopped fiber and the copolymer resin and extrudes the resultant blend through a 2 to 4 mm diameter die opening to avoid clogging and to align the carbon fibers in the extrusion direction within the matrix of copolymer resin to maximize the reinforcement provided by the carbon fiber. The carbon fiber content of the blend is 20-35 wt %, preferably 30-35 wt %.
U.S. Pat. No. 5,604,285 (Miyamori et al.) discloses the addition of both carbon fiber and metal powder to melt-processible fluoropolymer to obtain a reduced coefficient of friction while maintaining mechanical strength. The carbon fiber length is 80 to 3000 micrometers as determined from the disclosure of average fiber diameter of 10-30 micrometers and average aspect ratio of 8 to 300, preferably 20 to 300 micrometers. The fluoropolymer has its end groups stabilized either by reaction with ammonia to form amide end groups or by fluorination, which is known to form —CF3 end groups. Injection molding is also disclosed.
U.S. Pat. No. 5,705,120 (Ueno) discloses inadequate wettability between polytetrafluoroethylene and carbon fiber and addresses this shortcoming by using a heat-treated carbon fiber, called graphite fiber, having a length of 50 micrometers to 5 mm, and having a surface treatment characterized by either carbon/oxygen ratio or by the presence of halogen or both. The fluororesin mixtures disclosed in Ueno contain 1 to 60 wt % of the specially treated graphite fiber. Compression molding and extrusion and injection molding are disclosed as fabrication methods, noting the importance of uniform admixing of the fluororesin and fiber ingredients. Only compression molding is used in the Examples.
There is a need for melt-fabricable tetrafluoroethylene copolymers that have substantially greater creep resistance than provided by the copolymer itself and that can be made by economical manufacture.